Jumat, 25 Maret 2011

This is a video scan converter to display NTSC video signal into VGA monitor. Normally, the VGA monitor occupies most desk top space, so that everybody will be thinking that if the VGA monitor can be used for video monitor.
I tried to build a scan converter. Right photo shows my VGA monitor which displays TV. The VGA monitor (Nanao T560iJ) has two VGA inputs, BNCs and a D-sub, it can easy to change its inputs without any external video swicher. A small monitor at left end of the photo is a sub-monitor to be used during the VGA monitor is used for PC.
Now, difference between NTSC and VGA is shown in table 1. The horizontal refresh rate for VGA is just two times faster than NTSC, and vertical refresh rates are equal. Thus, only converting the horizontal refresh rate is sufficient for NTSC to VGA scan converter and its circuit diagram is very simple. If there is difference between vertical refresh rates such as PAL to VGA converter, it will require frame buffer, the circuit diagram will become complexity. I can't give any comment on PAL to VGA conversion.

Table 1. Refresh rate

Video signal

H sync

V sync

NTSC

15.734kHz

59.94Hz

VGA

31.469kHz

59.94Hz

Operation

Figure 1 shows the block diagram for the scan converter. Separated NTSC signal is decoded into RGB signals by the video deocder. The analog RGB signals are digitaized by video A-D converters and written into the line memories. The line memories are read at two times faster than write rate, same data will be read two times during one scan line is written. The timing diagram for this operation is shown in figure 2.
The timing clocks for each block is generated with phase locked loop which locks input horizontal frequency.

Figure 1. Block diagram for scan converter

Figure 2. Timing diagram for scan conversion

Building the circuit board

The scan converter has fast digital circuit and fast analog circuit. These should be separated from each other to prevent influence of digital noize from digital block to analog block. Especially, the phase locked loop must operate stably, otherwise the scan converter cannot be used for practical use. The phase locked loop is locked to input horizontal frequency stably, then adjust any other trimmer pots.